Consider the usage of your tiles. Is it in a high traffic area? Are you living with a clumsy family or in a area where it may be accident prone? Cracked tiles happen so you should consider it a possibility in your own home. This can lead to extra costs.
If you want custom tiles made, order a bit extra to plan for any small installation problems as well as damages later down the line. Putting in a new order for just a few custom tiles later on can cost you more since you are not getting the cost advantage of a larger order.
Some tiles are more fragile than others. Take into account the thickness of the tile as well as how large it is. Very thin tiles are obviously more prone to breakage. Very large tiles means you need to replace more square footage. If you accidentally drop, say–a toaster on a tile with smaller floors, you may end up only replacing one or two 3″ tiles versus a huge 12″ tile.